FYI - for others trying to understand this -
The verb "alcanzar" means different when used as a Transitive Verb (requires a direct object) or as an Intransitive Verb.
Transitive verb = "To reach"' ; ejemplo: eso va a alcanzarlo - That is going to reach it.
Intransitive verb = "To be sufficient/enough"; eso va a alcanzar
This one was hard, but when I thought of it more like "that (the food) will reach (all the people coming for dinner)" then it makes more sense - "that will be enough (food for everyone)." Rather than the interjection "that will be enough!" as in saying "stop it already! " The above commenter suggesting "that will suffice" helped a lot.
There is a caveat. ? The sentence has a double meaning and possibly should be avoided. (i) ... Eso va a alcanzar (ex. " He wanted a car and "- transitive ) - is going to achieve, to reach that goal; (ii) Intransitive - eso es suficiente. Unless Duolingo adds up something as a small preamble or introduction we'll be left in doubt.